Move GIT_CREDENTIAL_STORE_SCRIPT from variables volume to scripts configmap
MR: Pending
## Description
Followup from https://gitlab.com/gitlab-org/gitlab/-/merge_requests/182354#note_2481242318 on https://gitlab.com/gitlab-org/gitlab/-/merge_requests/182354+s
See TODO in `ee/lib/remote_development/workspace_operations/create/workspace_variables_builder.rb`:
```ruby
{
# TODO: We should be able to move this entry to the scripts volume.
# This script is set as the value of `credential.helper` below in `GIT_CONFIG_VALUE_0`
key: GIT_CREDENTIAL_STORE_SCRIPT_FILE_NAME,
value: GIT_CREDENTIAL_STORE_SCRIPT,
variable_type: FILE_TYPE,
workspace_id: workspace_id
},
```
## Acceptance criteria
- [ ] Script is moved from variables volume to scripts configmap volume
## Implementation plan
- Move it, make sure tests pass, and workspaces still function.
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issue